This invention relates to novel solder compositions or solder creams, and is particularly concerned with the provision of solder compositions especially adapted for soldering components in microelectronic circuits, and wherein the use of fluxes, particularly rosin type flux, is avoided.
A solder cream is a material that is capable of being applied to a substrate or surface in a specific pattern using screening or analogous methods which can subsequently undergo fusing to provide an electrical joint or interface commonly described as a solder joint. The solder cream generally consists of metallic particles of various alloys such as tin-lead, tin-lead-silver, etc., contained in a vehicle including a flux such as wood rosin, or derivatives thereof.
Currently, microelectronic circuits are made by surface mounting chip carriers and other components on ceramic and plastic substrates by vapor phase soldering. Circuits similar to those used on printed circuit boards are produed on the substrates. In certain instances, the circuit is located on one side of the substrate, with pads being provided in certain areas, other parts of the circuit being covered with an insulating material.
The solder cream having a paste-like consistency is applied, for example by means of a silk screen, to the pads on the circuit board. Thereafter, the electronic components are carefully positioned with their peripheral contacts on the solder cream-coated pads. When all of the components are thus in place, the board, with such components temporarily positioned and retained thereon by the solder cream, can be placed in a vapor reflow system and subjected to a sufficiently high temperature to cause the metal content of the solder cream to liquefy and the contacts of the electronic components to be fused and to adhere to the pads on the circuit board. Examples of solder compositions are described in U.S. Pat. No. 3,684,533 to Conwicke and U.S. Pat. No. 4,373,974 to Barajas.
However, solder creams presently utilized, such as those of the above patents, contain flux constituents which char during soldering and coat the circuits with electrically conductive matter, causing shorts if not completely removed in post-soldering cleaning.
Thus, fluxes generally contain rosins which are difficult to clean from substrates or circuit boards. If one attempts to reflow the solder using infra-red, the light frequencies are translucent to the flux, and tend to bake the rosin on the boards, making it extremely difficult to remove or clean the baked-on rosin residue. If vapor phase soldering is used to reflow the solfer, the tendency is to pre-bake the flux, creating the same undesirable residue which is difficult to remove. Further, where solvents are employed to remove such residues, the solvent vapors are hazardous in work areas.
Since the reflow soldering process has many advantages such as temperature stability under inert atmosphere conditions, it was sought to enhance the process to particularly avoid the above noted problems associated with deposition and removal of char residue from circuit boards. It was considered particularly desirable to replace the rosin type flux employed in solder creams used in electronic circuitry, with a material more compatible with certain liquid fluorinated compounds such as perfluorotriamyl amine, often used in vapor phase soldering as the vapor reflow medium for heating the solder cream to liquefy same.
It is accordingly an object of the present invention to provide an improved solder composition or solder cream which is especially adapted for soldering components in electronic circuitry, particularly microelectronic circuits.
Another object is the provision of a solder composition of the above type which avoids formation of residues formed on substrates or circuit boards following vapor phase soldering operations.
A still further object of the invention is to provide an efficient solder composition of the above type especially designed for use in vapor phase soldering operations and which utilizes a vehicle for the solder composition which is free of rosin or rosin type flux components, such vehicle being compatible particularly with certain fluorinated compounds employed as the heating medium in vapor phase soldering.
Yet another object is the provision of a vapor phase soldering process employing an improved solder composition of the above type, in conjunction with certain fluorinated compounds employed to provide the vapor phase reflow medium.